1. Field of the Invention
This invention relates to liquid-level measurement, and concerns in particular devices for detecting and measuring the level of liquid in a container.
2. Description of the Prior Art
It is in general known that level measurements of a liquid in a container may be obtained by observing the behaviour of a vibrating rod partially immersed in the liquid. In particular, it is known that the natural flexural resonance frequencies of the rod are dependent upon the length of rod above and below the liquid surface, and that the detection of resonance at different frequencies can thus be used to indicate liquid level.
In my U.S. Pat. No. 4,213,337 I have described a liquid-level sensing device comprising an elongate member for immersion in a liquid, flexure wave transmitting means for launching flexure waves along the member, flexure wave receiving means for receiving flexure waves launched by the transmitting means, phase detection means for obtaining a phase difference measurement between the launched and received waves, and indicator means for showing thereby the extent of immersion of the elongate member. The device may be used to measure the degree of immersion either directly, as a function of the transit time (as given by the phase difference) of fixed frequency flexure waves passing along the elongate member, or indirectly, as a function of the resonance frequency of the elongate member when stimulated by scanned frequency flexure waves.
The device of my aforementioned Patent, though useful, does nevertheless suffer from one particular minor drawback, in that when used to measure liquid level in the indirect, resonance, mode it is not truly linear--thus, a graph of true liquid level against actual resonance frequency is not a straight line. Indeed, the graph, though averaging a straight line overall, exhibits a regular, periodic, perturbation arising from the relative insensitivity of the resonance frequency when a resonance node is approximately in conjunction with the liquid surface.